“Frac sand” is a high-purity silica sand with very durable and very round grains. It is a crush-resistant material produced for use by the petroleum industry. It is used in the hydraulic fracturing process (known as “fracking”) to produce petroleum fluids, such as oil, natural gas and natural gas liquids from rock units that lack adequate pore space for these fluids to flow to a well.
Some subsurface rock formations, such as organic shale, contain large amounts of oil, natural gas or natural gas liquids that will not flow freely to a well. They will not flow to a well because the rock unit either lacks permeability (interconnected pore spaces) or the pore spaces in the rock are so small that these fluids can not flow through them.
The hydraulic fracturing process solves this problem by generating fractures in the rock. This is done by drilling a well into the rock, sealing the portion of the well in the petroleum-bearing zone, and pumping water under high pressure into that portion of the well. This water is generally treated with a chemicals and thickeners such as guar gum to create a viscous gel. This gel facilitates the water's ability to carry grains of frac sand in suspension.
Large pumps at the surface increase the water pressure in the sealed portion of the well until it is high enough to exceed the breaking point of the surrounding rocks. When their breaking point is reached, the rocks fracture suddenly and water rushes rapidly into the fractures, inflating them and extending them deeper into the rock. Billions of sand grains are carried deep into the fractures by this sudden rush of water. A few thousand tons of frac sand may be required to stimulate a single well.
When the pumps are turned off, the fractures deflate but cannot close completely because they are propped open by the frac sand. This only occurs if enough sand grains to resist the force of the closing fractures have been delivered into the rock.
The new fractures in the rock, propped open by the durable sand grains, form a network of pore space that allows petroleum fluids to flow out of the rock and into the well. Frac sand is known as a “proppant” because it props the fractures open.
Frac sand must meet very demanding specifications. Frac sand must usually comprise high-purity silica sand with a specific grain size perfectly matched to job requirements, with a spherical shape that enables it to be carried in hydraulic fracturing fluid with minimal turbulence, along with a durability to resist crushing forces of closing fractures. Frac sand is produced in a range of sizes from as small as 0.1 mm in diameter to over 2 mm in diameter depending upon customer specifications. Most of the frac sand used is between 0.4 and 0.8 mm in size.
Frac sand specifications are the responsibility in the USA of the American Petroleum Institute (API) and the current standard is API RP 56. These specifications are very demanding and, as a result, suitable deposits are limited. The limited availability of natural reserves which are suitable for frac sand production coupled with growing demand ensures a high price for any producers able to meet the API RP 56 frac sand specifications.
Frac sand is not used straight from the ground. It requires processing or refining to optimize its performance. After mining, the sand is cleaned in a washing plant to remove clay, silt and other fine contaminants. After washing, the sand is typically stacked in piles to allow the wash water to drain off. This operation is usually done outdoors and is restricted to times of the year when temperatures are above freezing. After the sand is drained, it is typically placed in an air dryer to remove all moisture. The dry grains are then screened to obtain specific size fractions for different customers. Sand that is not suitable for fracking may be separated and sold for other uses.
Some sand refining plants are located at the mine site. However, known sand refining plants are very expensive and time consuming to build and are usually very large. Therefore they are often shared by multiple mines. These plants are therefore often centrally located to several mines and the sand is delivered to the plant by truck, train or conveyer.